I, II, and IV, and the Immigrant Song from III. Houses of the Holy as well. Stuff after that, well... some rare gems, but not much. As soon as they start beginning every song with the name (In the eveniiiing... etc) is when my interest starts to wane.
The program director for my dad's radio station actually told me that Zeppelin wasn't a blues band. He'd only really heard Stairway to Heaven...